The Community Land Model (CLM) is the land component of the Community Earth System Model (CESM) and is used in several global and regional modeling systems. In this paper, we introduce model ...developments included in CLM version 5 (CLM5), which is the default land component for CESM2. We assess an ensemble of simulations, including prescribed and prognostic vegetation state, multiple forcing data sets, and CLM4, CLM4.5, and CLM5, against a range of metrics including from the International Land Model Benchmarking (ILAMBv2) package. CLM5 includes new and updated processes and parameterizations: (1) dynamic land units, (2) updated parameterizations and structure for hydrology and snow (spatially explicit soil depth, dry surface layer, revised groundwater scheme, revised canopy interception and canopy snow processes, updated fresh snow density, simple firn model, and Model for Scale Adaptive River Transport), (3) plant hydraulics and hydraulic redistribution, (4) revised nitrogen cycling (flexible leaf stoichiometry, leaf N optimization for photosynthesis, and carbon costs for plant nitrogen uptake), (5) global crop model with six crop types and time‐evolving irrigated areas and fertilization rates, (6) updated urban building energy, (7) carbon isotopes, and (8) updated stomatal physiology. New optional features include demographically structured dynamic vegetation model (Functionally Assembled Terrestrial Ecosystem Simulator), ozone damage to plants, and fire trace gas emissions coupling to the atmosphere. Conclusive establishment of improvement or degradation of individual variables or metrics is challenged by forcing uncertainty, parametric uncertainty, and model structural complexity, but the multivariate metrics presented here suggest a general broad improvement from CLM4 to CLM5.
Plain Language Summary
The Community Land Model (CLM) is the land component of the widely used Community Earth System Model (CESM). Here, we introduce model developments included in CLM version 5 (CLM5), the default land component for CESM2 which will be used for the Coupled Model Intercomparison Project (CMIP6). CLM5 includes many new and updated processes including (1) hydrology and snow features such as spatially explicit soil depth, canopy snow processes, a simple firn model, and a more mechanistic river model, (2) plant hydraulics and hydraulic redistribution, (3) revised nitrogen cycling with flexible leaf stoichiometry, leaf N optimization for photosynthesis, and carbon costs for plant nitrogen uptake, (4) expansion to six crop types (global) and time‐evolving irrigated areas and fertilization rates, (5) improved urban building energy model, and (6) carbon isotopes. New optional features include a demographically structured dynamic vegetation model, ozone damage to plants, and fire trace gas emissions coupling to the atmosphere. Model performance is generally improved for most assessed variables and metrics, though clear establishment of improvement or degradation is challenging due to model complexity as well as observational data limitations. Nonetheless, CLM5 is increasingly suited for research into a broad range of societally relevant scientific questions related to the terrestrial system.
Key Points
Updated Community Land Model has more hydrological and ecological process fidelity and more comprehensive representation of land management.
The model is systematically evaluated using International Land Model Benchmarking system and shows marked improvement over prior versions.
The Greenland ice sheet is one of the largest contributors to global mean sea-level rise today and is expected to continue to lose mass as the Arctic continues to warm. The two predominant mass loss ...mechanisms are increased surface meltwater run-off and mass loss associated with the retreat of marine-terminating outlet glaciers. In this paper we use a large ensemble of Greenland ice sheet models forced by output from a representative subset of the Coupled Model Intercomparison Project (CMIP5) global climate models to project ice sheet changes and sea-level rise contributions over the 21st century. The simulations are part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We estimate the sea-level contribution together with uncertainties due to future climate forcing, ice sheet model formulations and ocean forcing for the two greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results indicate that the Greenland ice sheet will continue to lose mass in both scenarios until 2100, with contributions of 90±50 and 32±17 mm to sea-level rise for RCP8.5 and RCP2.6, respectively. The largest mass loss is expected from the south-west of Greenland, which is governed by surface mass balance changes, continuing what is already observed today. Because the contributions are calculated against an unforced control experiment, these numbers do not include any committed mass loss, i.e. mass loss that would occur over the coming century if the climate forcing remained constant. Under RCP8.5 forcing, ice sheet model uncertainty explains an ensemble spread of 40 mm, while climate model uncertainty and ocean forcing uncertainty account for a spread of 36 and 19 mm, respectively. Apart from those formally derived uncertainty ranges, the largest gap in our knowledge is about the physical understanding and implementation of the calving process, i.e. the interaction of the ice sheet with the ocean.
We describe and evaluate version 2.1 of the Community Ice Sheet Model (CISM).
CISM is a parallel, 3-D thermomechanical model, written mainly in Fortran,
that solves equations for the momentum balance ...and the thickness and
temperature evolution of ice sheets. CISM's velocity solver incorporates a
hierarchy of Stokes flow approximations, including shallow-shelf,
depth-integrated higher order, and 3-D higher order. CISM also includes a
suite of test cases, links to third-party solver libraries, and
parameterizations of physical processes such as basal sliding, iceberg
calving, and sub-ice-shelf melting. The model has been verified for standard
test problems, including the Ice Sheet Model Intercomparison Project for
Higher-Order Models (ISMIP-HOM) experiments, and has participated in the
initMIP-Greenland initialization experiment. In
multimillennial simulations with modern climate forcing on a 4 km grid, CISM
reaches a steady state that is broadly consistent with observed flow patterns
of the Greenland ice sheet. CISM has been integrated into version 2.0 of the
Community Earth System Model, where it is being used for Greenland
simulations under past, present, and future climates. The code is open-source
with extensive documentation and remains under active development.
Projection of the contribution of ice sheets to sea level change as part of
the Coupled Model Intercomparison Project Phase 6 (CMIP6) takes the form
of simulations from coupled ice sheet–climate ...models and stand-alone ice
sheet models, overseen by the Ice Sheet Model Intercomparison Project for
CMIP6 (ISMIP6). This paper describes the experimental setup for
process-based sea level change projections to be performed with stand-alone
Greenland and Antarctic ice sheet models in the context of ISMIP6. The
ISMIP6 protocol relies on a suite of polar atmospheric and oceanic
CMIP-based forcing for ice sheet models, in order to explore the uncertainty
in projected sea level change due to future emissions scenarios, CMIP
models, ice sheet models, and parameterizations for ice–ocean interactions.
We describe here the approach taken for defining the suite of ISMIP6
stand-alone ice sheet simulations, document the experimental framework and
implementation, and present an overview of the ISMIP6 forcing to be
used by participating ice sheet modeling groups.
The Glimmer Community Ice Sheet Model (Glimmer-CISM) has been implemented in the Community Earth System Model (CESM). Glimmer-CISM is forced by a surface mass balance (SMB) computed in multiple ...elevation classes in the CESM land model and downscaled to the ice sheet grid. Ice sheet evolution is governed by the shallow-ice approximation with thermomechanical coupling and basal sliding. This paper describes and evaluates the initial model implementation for the Greenland Ice Sheet (GIS). The ice sheet model was spun up using the SMB from a coupled CESM simulation with preindustrial forcing. The model’s sensitivity to three key ice sheet parameters was explored by running an ensemble of 100 GIS simulations to quasi equilibrium and ranking each simulation based on multiple diagnostics. With reasonable parameter choices, the steady-state GIS geometry is broadly consistent with observations. The simulated ice sheet is too thick and extensive, however, in some marginal regions where the SMB is anomalously positive. The topranking simulations were continued using surface forcing from CESM simulations of the twentieth century (1850–2005) and twenty-first century (2005–2100, with RCP8.5 forcing). In these simulations the GIS loses mass, with a resulting global-mean sea level rise of 16mm during 1850–2005 and 60mm during 2005–2100. This mass loss is caused mainly by increased ablation near the ice sheet margin, offset by reduced ice discharge to the ocean. Projected sea level rise is sensitive to the initial geometry, showing the importance of realistic geometry in the spun-up ice sheet.
The Greenland Ice Sheet (GrIS) mass balance is examined with an Earth system/ice sheet model that interactively couples the GrIS to the broader Earth system. The simulation runs from 1850 to 2100, ...with historical and SSP5‐8.5 forcing. By the mid‐21st century, the cumulative GrIS contribution to global mean sea level rise (SLR) is 23 mm. During the second half of the 21st century, the surface mass balance becomes negative in all drainage basins, with an additional SLR contribution of 86 mm. The annual mean GrIS mass loss in the last two decades is 2.7‐mm sea level equivalent (SLE) year−1. The increased SLR contribution from the surface mass balance (3.1 mm SLE year−1) is partly offset by reduced ice discharge from thinning and retreat of outlet glaciers. The southern GrIS drainage basins contribute 73% of the mass loss in mid‐century but 55% by 2100, as surface runoff increases in the northern basins.
Plain Language Summary
The Greenland Ice Sheet (GrIS) gains mass at the surface from snowfall, and it loses mass from melting and runoff and from glacier calving at the ocean front. When these processes are in balance, the ice sheet does not contribute to global sea level change. Recent observations have shown that the ice sheet is losing mass and raising global mean sea level.
This study uses a global Earth system model that calculates ice flow of the GrIS, as well as processes in the atmosphere, ocean, land, and sea ice. For a modern reference, the model is forced with atmospheric greenhouse gas concentrations for the period 1850–2014. Next, the model is forced for the rest of the 21st century following the SSP5‐8.5 scenario to study how the GrIS and the Earth system respond to a worst‐case scenario.
By 2050, the GrIS loses mass that is equal to 23 mm of global mean sea level rise. During the second half of the 21st century, all regions of the GrIS lose mass because of increased surface melting and runoff, with the dry north playing a greater role. By 2100, the projected GrIS contribution to sea level rise is 109‐mm sea level equivalent.
Key Points
CESM2.1‐CISM2.1 simulates a 5.4 K global mean temperature increase and strong NAMOC weakening by 2100 in SSP5‐8.5 w.r.t. preindustrial
The Greenland Ice Sheet contributes 23 mm to global mean sea level rise by 2050 and 109 mm by 2100
The role of the northern basins becomes more important as surface runoff strongly increases during the second half of the 21st century
In Earth system models, terrestrial snow is usually modeled by the land surface component. In most cases, these snow models have been developed with an emphasis on seasonal snow. Questions about ...future sea level rise, however, prompt the need for a realistic representation of perennial snow, as snow processes play a key role in the mass balance of glaciers and ice sheets. Here we enhance realism of modeled polar snow in the Community Land Model (CLM), the land component of the Community Earth System Model (CESM), by implementing (1) new parametrizations for fresh snow density, destructive metamorphism, and compaction by overburden pressure, (2) by allowing for deeper snow packs, and (3) by introducing drifting snow compaction, with a focus on the ice sheet interior. Comparison with Greenlandic and Antarctic snow density observations show that the new physics improve model skill in predicting firn and near‐surface density in the absence of melt. Moreover, compensating biases are removed and spurious subsurface melt rates at ice sheets are eliminated. The deeper snow pack enhances refreezing and allows for deeper percolation, raising ice temperatures up to 15°C above the skin temperature.
Key Points
Near‐surface snow density modulates heat transport and, indirectly, melt, and should thus be realistically modeled over ice sheets
New snow physics and parameters show a significant improvement with respect to snow density observations, both at the surface and at depth
Deep ice temperatures rise by several degrees due to the deepening of the snow pack, and the associated deeper percolation
Abstract
Firn (compressed snow) covers approximately 90
$$\%$$
%
of the Greenland ice sheet (GrIS) and currently retains about half of rain and meltwater through refreezing, reducing runoff and ...subsequent mass loss. The loss of firn could mark a tipping point for sustained GrIS mass loss, since decades to centuries of cold summers would be required to rebuild the firn buffer. Here we estimate the warming required for GrIS firn to reach peak refreezing, using 51 climate simulations statistically downscaled to 1 km resolution, that project the long-term firn layer evolution under multiple emission scenarios (1850–2300). We predict that refreezing stabilises under low warming scenarios, whereas under extreme warming, refreezing could peak and permanently decline starting in southwest Greenland by 2100, and further expanding GrIS-wide in the early 22
$^{{nd}}$$
n
d
century. After passing this peak, the GrIS contribution to global sea level rise would increase over twenty-fold compared to the last three decades.
The sea level contribution of the Antarctic ice sheet
constitutes a large uncertainty in future sea level projections. Here we
apply a linear response theory approach to 16 state-of-the-art ice sheet
...models to estimate the Antarctic ice sheet contribution from basal ice shelf
melting within the 21st century. The purpose of this computation is to
estimate the uncertainty of Antarctica's future contribution to global sea
level rise that arises from large uncertainty in the oceanic forcing and the
associated ice shelf melting. Ice shelf melting is considered to be a major
if not the largest perturbation of the ice sheet's flow into the ocean.
However, by computing only the sea level contribution in response to ice
shelf melting, our study is neglecting a number of processes such as
surface-mass-balance-related contributions. In assuming linear response
theory, we are able to capture complex temporal responses of the ice sheets,
but we neglect any self-dampening or self-amplifying processes. This is
particularly relevant in situations in which an instability is dominating the
ice loss. The results obtained here are thus relevant, in particular wherever the
ice loss is dominated by the forcing as opposed to an internal instability,
for example in strong ocean warming scenarios. In order to allow for
comparison the methodology was chosen to be exactly the same as in an
earlier study (Levermann
et al., 2014) but with 16 instead of 5 ice sheet models. We include
uncertainty in the atmospheric warming response to carbon emissions (full
range of CMIP5 climate model sensitivities), uncertainty in the oceanic
transport to the Southern Ocean (obtained from the time-delayed and scaled
oceanic subsurface warming in CMIP5 models in relation to the global mean
surface warming), and the observed range of responses of basal ice shelf
melting to oceanic warming outside the ice shelf cavity. This uncertainty in
basal ice shelf melting is then convoluted with the linear response
functions of each of the 16 ice sheet models to obtain the ice flow response
to the individual global warming path. The model median for the
observational period from 1992 to 2017 of the ice loss due to basal ice
shelf melting is 10.2 mm, with a likely range between 5.2 and 21.3 mm. For
the same period the Antarctic ice sheet lost mass equivalent to 7.4 mm of
global sea level rise, with a standard deviation of 3.7 mm (Shepherd et al., 2018) including all processes,
especially surface-mass-balance changes. For the unabated warming path,
Representative Concentration Pathway 8.5 (RCP8.5), we obtain a median contribution of the Antarctic ice sheet to
global mean sea level rise from basal ice shelf melting within the 21st
century of 17 cm, with a likely range (66th percentile around the mean) between
9 and 36 cm and a very likely range (90th percentile around the mean)
between 6 and 58 cm. For the RCP2.6 warming path, which will keep the
global mean temperature below 2 ∘C of global warming and is thus
consistent with the Paris Climate Agreement, the procedure yields a median of
13 cm of global mean sea level contribution. The likely range for the
RCP2.6 scenario is between 7 and 24 cm, and the very likely range is
between 4 and 37 cm. The structural uncertainties in the method do not
allow for an interpretation of any higher uncertainty percentiles. We provide
projections for the five Antarctic regions and for each model and each
scenario separately. The rate of sea level contribution is highest under
the RCP8.5 scenario. The maximum within the 21st century of the median
value is 4 cm per decade, with a likely range between 2 and 9 cm per decade
and a very likely range between 1 and 14 cm per decade.
The future retreat rate for marine-based regions of the Antarctic Ice Sheet is one of the largest uncertainties in sea-level projections. The Ice Sheet Model Intercomparison Project for CMIP6 ...(ISMIP6) aims to improve projections and quantify uncertainties by running an ensemble of ice sheet models with
atmosphere and ocean forcing derived from global climate models.
Here, the Community Ice Sheet Model (CISM) is used to run ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution. Using multiple combinations of sub-ice-shelf melt parameterizations and calibrations, CISM is spun up to steady state over many millennia. During the spin-up, basal friction parameters and basin-scale thermal forcing corrections are adjusted to optimize agreement with the observed ice thickness.
The model is then run forward for 550 years, from 1950–2500, applying ocean thermal forcing anomalies from six climate models. In all simulations, the ocean forcing triggers long-term retreat of the West Antarctic Ice Sheet, especially in the Filchner–Ronne and Ross sectors. Mass loss accelerates late in the 21st century and then rises steadily for several centuries without leveling off.
The resulting ocean-forced sea-level rise at year 2500 varies from about 150 to 1300 mm, depending on the melt scheme and ocean forcing.
Further experiments show relatively high sensitivity to the basal friction law, moderate sensitivity to grid resolution and the prescribed collapse of small ice shelves, and low sensitivity to the stress-balance approximation.
The Amundsen sector exhibits threshold behavior, with modest retreat under many parameter settings but complete collapse under some combinations of low basal friction and high thermal forcing anomalies.
Large uncertainties remain, as a result of parameterized sub-shelf melt rates,
simplified treatments of calving and basal friction, and the lack of ice–ocean coupling.
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